Cochlear models: evidence in support of mechanical nonlinearity and a second filter (a review).

We have studied properties of a nonlinear one-dimensional model for motion of the basilar membrane. The model is able to reproduce a variety of physiological and psychophysical effects, and in some instances predictions of the model have been subssequently confirmed. We regard the agreement between model and nature as evidence that the major features of the model are qualitatively correct. Areas of agreement include generation and propagation of two-toine distortion products, two-tone suppression, and Zwicker's psychophysical masking period patterns. In order to reproduce all of the above effects, the model must contain the following major features: (1) A mechanical nonlinearity. This nonlinearity is asymmetric, with membrane damping increasing during the same phase of membrane motion that elicits response activity in primary auditory fibers. (2) An additional stage of sharpening, a 'second filter', between the physical variable controlling mechanical nonlinearity and the physical variable controlling neural excitation.